To form a long metal product, roll forming and press forming are mainly used. In the press forming, a workpiece is basically subjected only to two-dimensional deformation in cross section, thereby easily obtaining high product dimension precision, with less additional deformation and residual stress; however, the equipment cost including dies is high, the productivity is low, and there is product length limitation.
In the roll forming, since the edge of a workpiece is hard to pass through a large number of forming roll stands, it is difficult to use a sheet material; however, continuous production using a coiled material is enabled, so that the product length limitation is less, the productivity is high, and the equipment cost is less expensive than the press forming. However, forming rolls as rotors cannot be large due to production ability and cost limitation, additional deformation occurs in a workpiece subjected to three-dimensional deformation typified by twisting thereof onto the rolls, and further, the resistance in the advancing direction is large due to the twisting, thereby making the necessary driving energy large. In addition, the circumferential speed difference in the contact region between the forming rolls and a workpiece is great; therefore, the surface quality becomes a problem often due to product damage caused by the relative slip between both. The contact region between the forming rolls and a workpiece is small, so that the surface pressure between both becomes higher, resulting in the rolls becoming significantly worn, together with the circumferential speed difference.
Typically, an electric resistance welded pipe is manufactured by, with the use of the forming rolls, being subjected to a pre-step of uncoiling a coiled material to supply it to a forming step, an early-stage forming step performed by breakdown rolls, cluster rolls, and fin pass rolls, a welding step of welding the opposite edges of the material at e.g., a high frequency, a sizing step of correcting circularity and straightness of the pipe, and a cutting step of cutting the manufactured pipe to a predetermined length.
For instance, in the breakdown step, as a forming method showing a forming process from a raw plate into a pipe, a roll flower showing an edge track process from a raw plate into a pipe, of an edge bending method in which a material edge track is a cycloid curve, a center bending method in which the track is an involute curve, a circular bending method, a forming method combining these, or a double bending method is appropriately selected; however, basically, a pair of upper and lower convex and concave rolls and side rolls are used to hold a workpiece from the inner and outer surfaces thereof, thereby forming it into a desired cross-sectional shape.